This invention relates to charged particle beam application technologies, and more specifically, to charged particle beam apparatuses, such as a scanning electron microscope (SEM) equipped with an aberration corrector, an electron beam semiconductor inspection system, a critical dimension measurement SEM, and a focused ion beam apparatus.
Since the scanning electron microscope can observe the surface of an object with resolution higher than that of the optical microscope, it is widely used not only as apparatuses for material research but also as industrial apparatuses, such as for dimension measurement of a pattern on a semiconductor wafer that is being advanced in miniaturization in recent years and inspection of surface foreign matters. In specimens (wafers) of the semiconductor industry that use insulators, high resolution of a few nm has become required at a low acceleration voltage of 1 kV or less at which observation can be performed without electrifying the insulators. Since the resolution of the SEM depends on how small an electron beam is focused on a specimen plane, it is determined by diffraction aberration, chromatic aberration and spherical aberration of electron lenses, etc. in addition to the size of an electron source that is reduction imaged by the lenses. Higher-resolution of the SEM has so far been attained by contrivance of an electron optical system, especially by reducing the aberrations by enlarging a reduction ratio of the electron source and optimizing the shape of an objective lens by combining an accelerating electric field and a decelerating electric field.
However, Scherzer proved that an objective lens having rotational symmetry about the optical axis can reduce neither spherical aberration nor chromatic aberration to zero, and accordingly these conventional methods have restrictions in achieving higher resolution from the viewpoint of dimensions of a shape, processing accuracy, material, and a withstand voltage. Then, a method of canceling the aberration of an objective lens with a chromatic and spherical aberration corrector that combined the quadrupole and the octupole was proposed (for example, refer to “H. Rose, Optik 33(1971), pp. 1-24”). In 1995, an SEM equipped with the aberration corrector has been put to practical use by Zach et al. (for example, refer to “J. Zach and M. Haider, Nuclear Instruments and Methods in Physics Research A363 (1995), pp. 316 to 325)”).
FIG. 4 shows a schematic diagram of the SEM electron optical system that includes the aberration corrector. Inside the aberration corrector, due to an effect of four-stage quadrupole fields, electrons travel on mutually different fundamental rays in two directions (an x-y coordinate system that rotates together with rotation of fundamental rays about the optical axis caused by a magnetic field lens) perpendicular to the optical axis (z-axis). They are called a fundamental ray x and a fundamental ray y. For example, the SEM electron optical system is so configured that the fundamental ray x forms a line image at a position of a second stage and the fundamental ray y forms a line image at a position of a third stage. If the quadrupole electric field and the quadrupole magnetic field are superposed so that force acting on electrons at the second stage and the third stage each having this line image may become constant, chromatic aberration of the system can be controlled independently in the x-direction and in the y-direction without changing the fundamental rays (for example, refer to “J. Zach and M. Haider, Nuclear Instruments and Methods in Physics Research A363 (1995), pp. 316-325,” and “S. Uno, K. Honda, N. Nakamura, M. Matsuya, and J. Zach, Optik 116(2005), pp. 438-448”).